Autothrottles control thrust on an aircraft and are used in many sections of flight, including take-off, cruise, and landing. During landing, the autothrottle retards throttles so that airspeed and rate of descent are reduced. The aircraft then descends at a rate that results in a safe and gentle touch-down.
Typically, autothrottle retard is initiated once the aircraft reaches a predetermined height above the runway. Determining an appropriate height can be important. If retard initiation height is too low, then the plane may descend too rapidly. If initiation height is too high, then the plane may stall. Either scenario may lead to significant structural damage and fatalities.
The retard initiation height is usually fixed for a given aircraft and, for example, may be around 50 feet above the runway. A fixed height may not be appropriate for all landings, increasing the risk that pilots may need to take evasive maneuvers to land a plane safely.